Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor formed using two or more kinds of atoms bonded together. In general, metal oxides become insulators. However, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are metal oxides which have semiconductor characteristics. A thin film transistor in which a transparent semiconductor layer which is formed using such a metal oxide serves as a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
Further, not only one-element oxides but also multi-element oxides are known as metal oxides. For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (also referred to as a “TFT”) (Patent Document 5 and Non-Patent Documents 5 and 6).
In addition, a method for manufacturing a transistor using an oxide semiconductor in which the electric resistance of the oxide semiconductor is reduced by making the oxide semiconductor include hydrogen is proposed. For example, in Patent Document 6, a method in which hydrogen is added to a source electrode and a drain electrode and hydrogen included in the source electrode and the drain electrode is diffused into an oxide semiconductor is disclosed.